1. Field of the Invention
The invention relates to medical diagnostic systems and methods. In particular, the invention relates to a system and method for obtaining high-resolution encephalographs.
2. Related Art
The information contained in this section relates to the background of the art of the present invention without any admission as to whether or not it legally constitutes prior art.
The following is a list of articles relating to various diagnostic techniques contemplated for assessing brain functions, as follows:                Ahonen, A. I., Hamalainen, M. S., Kajola, M. J., Knuutila, J. E. T., Laine, P. P., Lounasmaa, O. V., Parkkonen, L. T., Simola, J. T., and Tesche, C. D. 122 channel SQUID instrument for investigating the magnetic signals from the human brain. Physica Scripta, 1993, T49: 198–205;        Barth, D. S., Sutherling, W., Broffman, J., and Beatty, J. Magnetic localization of a dipolar current source implanted in a sphere and a human cranium. Electroenceph. clin. Neurophysiol., 1986, 63: 260–273;        Buchanan, D. S., Crum, D. B., Cox, D., & Wikswo, J. P. jr. MicroSQUID: A close-spaced four channel magnetometer. In S. J. Williamson et al., (eds.), Advances in Biomagnetism, Plenum Press, New York, 1989, pp. 677–679;        Curio, G., Mackert, B.-M., Abraham-Fuchs, K., and Härer, W. (1994) High-frequency activity (600 Hz) evoked in the human primary somatosensory cortex: a survery of electric and magnetic recordings. In C. Patev et al., eds. Oscillatory Event-Related Brain Dynamics, Penum Press, New York, pp. 205–218;        Curio, G., Mackert, B.-M., Burghoffm M., Koetitz, R., Abraham-Fuchs, K., and Härer, W. (1994) Localization of evoked neuromagnetic 600 Hz activity in the cerebral somatosensory system. Electroenceph. clin. Neurophysiol., 91: 483–487;        De Weerd, A. W. Atlas of EEG in the first months of life. El Sevier, New York, 1995;        Dreyfus-Brisac, C. The electroencephalogram of the premature infant and full-term newborn: normal and abnormal development of waking and sleeping patterns. In P. Kellaway and I. Petersén (eds.), Neurological and electroencephalographic correlative studies in infancy. Grune and Stratton, New York, 1964, pp. 186–207;        Dreyfus-Brisac, C. The electroencephalogram of the premature infant. World Neurol., 1962, 3: 5–15;        Dreyfus-Brisac, C., Samson, D., Blanc, C., and Monod, N. L'electroencéphalogramme de l'enfant normal de moins de 3 ans. Etud. néo-natal. 1958, 7: 143–175;        Emerson, R. G., Sgro, J. A., Pedley, T. A., Hauser, W. A. (1988) State-dependent changes in the N20 component of the median nerve somatosensory evoked potential. Neurology, 38: 64–68.        Erasmie, U., & Ringertz, H. Normal width of cranial sutures in the neonate and infant. Acta Radiol. Diagnosis, 1976, 17: 565–572;        Geselowitz, D. B. On the magnetic field generated outside an inhomogeneous volume conductor by internal current sources. IEEE Trans. Mag., 1970, 6: 346–347;        Gevins, A., Le, J., Leong, H., McEvoy, L. K., and Smith, M. E. (1999) Deblurring. J. Clin. Neurophysiol. 16: 204–213;        Gevins, A., Le, J., Martin, N. K., Brickett, P., Desmond, J., and Reutter, B. High resolution EEG: 124-channel recording, spatial deblurring and MRI integration methods. Electroenceph. clin. Neurophysiol., 1994, 90: 337–358;        Gobbelé, R., Buchner, H., and Curio, G. (1998) High-frequency (600 Hz) SEP activities originating in the subcortical and cortical human somatosensory system. Electroenceph. clin. Neurophysiol., 108: 182–189;        Goff, W. R., Allison, T., and Vaughan, H. G., Jr. (1978) The functional neuroanatomy of event related potentials. In: Event-related brain potentials in man. E. Callaway, P. Tueting, and S. H. Koslow (Eds.), Academic Press, New York San Francisco London, pp. 1–79;        Grynszpan, F. and Geselowitz, D. B. (1973) Model studies for the magnetocardiogram. Biophys. J., 13: 911–925;        Hämälainen, M. S., and llmoniemi, R. (1994) Interpreting magnetic fields of the brain: minimum norm estimates. Med. Biol. Eng. Comp., 32:35–42;        Hämälainen, M. S., and Sarvas, J. (1989) Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data. IEEE Trans. Biomed. Eng., 36:165–171;        Hämäläinen, M., and Sarvas, J. Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data. IEEE Trans. Biomed. Eng., 1989, 36: 165–171;        Hansman, C. F. (1966) Growth of interorbital distance and skull thickness as observed in roentgenographic measurements. Radiology, 86:87–96;        Hansman, C. F. Growth of interorbital distance and skull thickness as observed in roentgenographic measurements. Radiology, 1966, 86: 87–96;        Hashimoto, I., Mashiko, T., and Imada, T. (1996a) Somatic evoked high-frequency magnetic oscillations reflect activity of inhibitory interneurons in the human somatosensory cortex. Electroenceph. clin. Neurophysiol., 100:189–203;        Hashimoto, I., Papuashvili, N., Xu, C. and Okada, Y. C. (1996b) Neuronal activities from a deep subcortical structure can be detected magnetically outside the brain in the porcine preparation. Neurosci. Lett. 206:25–28;        Haueisen, J., Heuer, T., Nowak, H., Liepert, J., Weiller, C., Okada, Y. C., and Curio, G. (2000a) The influence of lorazepam on somatosensory evoked fast frequency (600 Hz) activity in MEG. Brain Res. in press;         Haueisen, J., Schack, B., Meier, T., Nowak, H., Weiller, C., Curio, G., and Okada, Y. C. (2000b) Time-frequency analysis of somatosensory evoked short latency cortical activity in MEG. To be submitted to Clinical Neurophysiology;        Humphrey, D. R. (1968a) Re-analysis of the antidromic cortical response. I. potentials evoked by stimulation of the isolated pyramidal tract. Electroenceph. clin. Neurophysiol., 24:116–129;        Humphrey, D. R. (1968b) Re-analysis of the antidromic cortical response. II. on the contribution of cell discharge and PSPs to the evoked potentials. Electroenceph. clin. Neurophysiol., 25:421–442;        Kaufman, L., Okada, Y., Brenner, D., and Williamson, S. J. (1981) On the relation between somatic evoked potentials and fields. Int. J. Neurosci., 15: 223–239;        Le, J., Menon, V., and Gevins, A. Local estimate of surface Laplacian derivation on a realistically shaped scalp surface and its performance on noisy data. Electroenceph. clin. Neurophysiol., 1994, 92: 433–441;        Lusted, L. B., and Keats, T. E. Atlas of roentgenographic measurement. Year Book Publishers, Chicago, 1978;        Mackert, B.-M., Weisenbach, S., Nolte, G., and Curio, G. (2000) Rapid recovery (20 ms) of human 600 Hz electroencephalographic wavelets after double stimulations of sensory nerves. Neurosci. Lett., 286:83–86;        Okada Y C, Lähteenmäki A, Xu C (1999a) Comparison of MEG and EEG on the basis of somatic evoked responses elicited by stimulation of the snout in the juvenile swine. Clin Neurophysiol 110:214–229;        Okada Y C, Lahteenmaki A, Xu C (1999b) Experimental analysis of distortion of magnetoencephalography signals by the skull. Clin Neurophysiol 110:230–238;        Okada Y C, Shah B, Huang J-C (1994) Ferromagnetic high-permeability alloy alone can provide sufficient low-frequency and eddy-current shieldings for biomagnetic measurements. IEEE Trans Biomed Eng 41:688–697;        Okada, Y. C., Shah, B. and Huang, J.-C. (1994) Ferromagnetic high-permeability alloy alone can provide sufficient low-frequency and eddy-current shieldings for biomagnetic measurements. IEEE Trans. BME, 41: 688–697;        Roark, R. J. and Young, W. C. Formulas for stress and strain. McGraw-Hill, New York, 1975;        Sunshine, P. Epidemiology of perinatal asphyxia. In: D. K. Stevenson and P. Sunshine, (eds.), Fetal and neonatal brain injury: Mechanisms, management and the risks of practice. Oxford Univ. Press, New York, 1997, pp. 3–23;        Sunshine, R. (1997) Epidemiology of perinatal asphyxia. In: D. K. Stevenson and P. Sunshine, (eds.), Fetal and neonatal brain injury: Mechanisms, management and the risks of practice. Oxford Univ. Press, New York, pp. 3–23;        Tharp, B. Use of the electroencephalogram in assessing acute brain damage in the newborn. In: D. K. Stevenson and P. Sunshine, (eds.), Fetal and neonatal brain injury: Mechanisms, management and the risks of practice. Oxford Univ. Press, New York, 1997, pp. 287–301;        Volpe, J. J. Neurology of the new born. W. B. Sanders, Philadelphia, Pa., 2000; and        Yamada, T., Kameyama, S., Fuchigami, Y., Nakazumi, Y., Dickens, Q. S., and Kimura, J. (1988) Changes of short latency somatosensory evoked potential in sleep. Electroenceph. clin. Neurophysiol., 70: 126–136. The foregoing articles are each incorporated herein by reference.        
The need for finding useful diagnostic techniques is becoming increasingly urgent today in assessing brain functions of infants. With advances in medicine, more and more pre- and full-term newborns survive even with neurological disabilities (Sunshine, 1997; Volpe, 2000). According to Sunshine (1997), the number of newborns with neurological impairments is quite large. The incidence of perinatal asphyxia is between about 2/1000 and about 47/1000. Between about 4% and about 26% of those newborns who survive such an event will have severe neurological deficits. The incidence of hypoxemic-ischemic encephalopathy in term or near-term infants is between about 3/1000 and 8/1000. Handicapped survivors may be as high as about 42% in such cases. The incidence of infants with neonatal seizures is between about 2/1000 and about 9/1000. The incidence of handicaps in the survivors is between about 11% and about 50%. The incidence of moderate-to-severe cerebral palsy in infants who survive the neonatal period is between about 1/1000 and 3/1000. The prevalence of severe mental retardation is between about 3/1000 and about 4/1000 school-age children. The incidence of mild mental retardation is between about 23/1000 and about 30/1000 in the same population. According to Volpe (2000), the percentage of preterm infants with proven periventricular white matter injury is about 45% for those with birth weight of less than about 1500 g, about 38% for those with gestational age of less than about 33 weeks and about 24% for those with gestational age of less than about 38 weeks. The percentage of asphyxiated term infants with some form of central nervous system injury is as high as about 62%, a common form of the injury being the parasagittal cerebral injury. Infants with germinal matrix hemorrhage is between about 23% and about 32% of all births delivered through the vaginal route when the delivery lasts more than six hours.
Survival of neurologically impaired neonates raises an important responsibility for the health care community in this country. Currently, electroencephalography (EEG) is used to monitor electrical activity of the brain of newborns (Sunshine, 1997). The use of EEG for perinatal monitoring was started in the late 1950's (Dreyfus-Brisac et al., 1958; Dreyfus-Brisac, 1962, 1964). Its use is increasing in recent years due to its usefulness in staging the development of the nervous system, in detecting the presence of hypoxic and intracranial injuries, in providing the prognosis of recovery and in differential diagnosis of seizures from non-seizures in paroxysmal motor behavior (Tharp, 1997; de Weerd, 1995). The staging is useful in detecting a delay or an arrest in brain development. The waveforms and spatial topography such as hemispheric asymmetry of spontaneous EEG are also useful for detecting the presence of a tumor or a necrotic area in the brain.
In order for magnatoencephalography (MEG) to become useful as a clinical electrophysiological monitoring technique, complementing EEG, it may be desirable for certain applications to have a MEG instrument which may be different from the conventional whole-head MEG instruments. In this regard, to be competitive with EEG instrumentation, a useful MEG instrument must be functional in any ordinary clinical rooms without any special cumbersome electromagnetic shielding such, for example, as a large and expensive, special purpose magnetically shielded room being currently used when conventional MEG techniques are employed.
Prior known MEG systems (such as those manufactured by Canadian Thin Films or CTF, Vancouver, Canada, 4-D Neuroimaging, San Diego, Calif., and Helsinki, Finland) are relatively large and heavy, and are used mostly in an expensive magnetically shielded room.
In order to use an MEG system to detect infant brainwaves with a sufficient high resolution, it would be desirable to be able to position the MEG sensors in very close proximity to the head of the infant patient. This would necessitate maintaining such close spacing between the MEG sensors and the patient's head without altering the spacing to any substantial extent during the use of this system.
The spacing between the patient's head and the sensor should be precisely maintained within reasonable tolerances for at least some applications. Due to the different coefficients of thermal expansion of the various components, this gap or spacing between the head and the sensors may be difficult to maintain in the presence of temperature changes for certain applications. Such temperature changes occur, for example, when the SQUID dewar warms from cryogenic temperatures toward room temperature. Also, when the dewar is cooled to its cryogenic temperatures, the components tend to change dimensionally for some applications.